Lava balls are formed from pieces
of breakdown that float and roll along in
a stream of moving lava, accreting mass as they move. It is much like a
snowball rolling downhill and growing larger as more snow sticks to it.Lava balls tend to roll along until being wedged at a
constriction, or often, are stuck to a ceiling during a surge in the flow.
They may get quite large, as shown in the second photo. Sometimes they
are lifted by upwelling lava through a window
or skylight and are deposited around the edges
before the lava drains back in, as in the third photo.